Help with making an osciloscope on NIOS II

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Is there any easy way to write externally DATA into SDRAM? or on_chip_ram?

I thought DMA should be used for this, but DMA can only access internal (maybe export ports?) registers.

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DMA can be indeed used for this.

The problem, as I stated before, is that sdram can't ensure the required throughput.

One reason is strictly connected with physical sdram access mode, which is not a simple rw/rd with a deterministic access time like sram: latencies due to row and column selection are involved; row precharge and possibly a refresh cycle can further delay the transaction.

The second reason is that sdram is usually shared with other devices, so the dma transfer can be delayed because someone else is accessing sdram when you need it.

That's why you need a dedicated buffer or fifo to store adc data at fast rate.